Is there any way to make random areas of a mesh trasparent?
Create a texture (like clouds, voronoi or musgrave or even an image texture) and have that texture control the mixture between the base cloud shader and a transparent one. Additionally, you can use a color ramp to control the contrast on the texture.



A more computationally intensive approach to making clouds, would be to use a texture to control the density of a volume scatter shader connected to the volume socket of the material.